ENERGETIC EFFICIENCY OF REPRODUCTION - THE BENEFITS OF ASYNCHRONOUS HATCHING FOR AMERICAN KESTRELS

1. The peak load and sibling rivalry hypotheses for hatching asynchron y in birds predict that the pattern of parental food provisioning shou ld differ between synchronous and asynchronous broods. Monitoring pare ntal effort is also an important step towards understanding the life h istory consequences of asynchrony. 2. In 1989-91, we used electronic e vent recorders at 57 nestboxes to record the number of visits American kestrels (Falco sparverius) made to experimentally synchronous and as ynchronous broods during the nestling period. In another experiment, w e recorded the prey provisioning by parents to broods supplemented wit h food. Nestlings were weighed every third day until fledging. 3. In a ll three years, parents made more visits to synchronous nests than to asynchronous ones. Provisioning rates did not differ between these two brood types when the nestlings were young, but differences in cumulat ive provisioning rates of up to 31 % were apparent by age 25 d. Parent s responded to supplemental food by decreasing the number of visits to the nest. 4. Despite the higher number of parental visits, the mass o f synchronous broods at fledging was less than asynchronous broods in all 3 years. 5. Synchronous broods require more energy to rear than as ynchronous broods with the same number of young. This is consistent wi th the sibling rivalry hypothesis, but not with the peak load hypothes is. The extra energetic cost of synchronous broods may be exacerbated when food is scarce.